They catch a diver’s eye in the same way a discarded plastic bag or yogurt pot might. An unexpected glimmer in the gloom of a typical offshore winter dive. But on closer inspection, the object is moving. It’s white with black speckles, like a melted Friesian cow, and turns out to be dragging a familiar item around on its ‘back’.
What is a common whelk?
A common whelk or ‘buckie’, Buccinum undatum, this sea snail, or at least its shell, is a common sight on almost any shoreline at this time of the year. In fact, it’s our largest gastropod, whose dull creamy shell can grow to be 10-11cm long. But it’s not the adult snail that fascinates as much as another related item you can find beachcombing this winter.
What are common whelk egg cases?
The sea wash ball, also known as ‘fisherman’s soap’, ‘egg cloud’ or common whelk egg case, is a strange, off-white cluster of flattish capsules – imagine a bundled-up sheet of bubble wrap. They’re the egg clusters of the common whelk and while they’re a constituent part of almost any strandline at any time of the year, November to January is the whelk’s breeding season.
Somewhere below the low tideline they amass. Quite how this frenzy of egg-laying starts is a mystery, but once they get going, other whelks quickly join in, perhaps sniffing out tell-tale odours in the water. The whelks are hermaphrodite, so this clustered spawning makes sense, because each snail can release both eggs and sperm, and they can all fertilise each other.
With each snail engaged in a slow-motion production line of egg capsules, each clump or cloud takes anything up to a week or so to complete. Sometimes they’re produced by a solitary animal but sometimes several contribute to form a single mass – a behaviour that explains the size differences you see in the egg clusters that wash up on the shore.
When you next find a sea wash ball, have a close look. Each of the ‘bubbles’ is not an egg, as is often thought, but an egg case. At this time of the year, many get torn from their anchorages and it’s often possible to find an egg case containing living occupants (live egg masses tend to have a more yellowy colouration).
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Hold an egg case up to the light (or use a torch to backlight them) and you should be able to make out the silhouettes of the developing snails within. Each case is packed with minuscule eggs and, depending on the size of the snails that laid them, can contain anything from 80 eggs up to several thousand.
Only about one per cent of them hatch into snails. Within each egg case they go through the various larval stages, safe from the outside world. Once the juveniles are 1-3mm long, they’re close to hatching and can easily be seen in the eggs. The surplus eggs aren’t wasted; they’re ‘nurse eggs’ to be consumed by the baby snails – an important part of their nutrition that allows them to hatch as fully formed, active snails.
Most egg cases washed up on the shoreline contain dehydrated juveniles and give a rather sad rattle when shaken. If you do discover a wash ball containing live whelks, however, give them a chance and return them to the sea!
Main image: Sarah Smith / Egg cases - Common Whelk
